1. Field of the Invention
The present invention relates to a method of solidifying in aspect the radioactive waste originating from nuclear power plants and other atomic energy facilities, as well as to asphalt-solidified radioactive wastes that are attained by this method and which have improved shape stability at elevated temperatures. More particularly, the present invention relates to a method by which liquid waste from a nuclear reactor that is in the form of an aqueous solution containing sodium sulfate or sodium borate as a chief component is concentrated by evaporation and mixed with hot asphalt to solidify solid components contained in the liquid waste in asphalt, the method being characterized by using straight asphalt that has a predetermined amount of a low-molecular weight polyolefin incorporated therein. The asphalt-solidified solid components of liquid waste obtained by the method of the present invention has improved stability in shape at elevated temperatures.
2. Description of the Prior Art
Mixing radioactive wastes with asphalt and storing the resulting solidified product in repositories has already been commercialized as a method of waste management in nuclear reactors. According to this method, the liquid waste from a nuclear reactor is concentrated in an evaporator and further dehydrated into a solidified state while it is mixed with concentrate or asphalt. While various methods of disposal have been proposed, the most common technique used today is to lay up the solidified waste in an underground through in a limited area and covered on top with soil so that it is confined below the ground level.
The radioactive wastes can be solidified in asphalt include liquid radioactive wastes containing sodium sulfate or sodium borate, those containing suspended solids or various salts, laundry liquid wastes, and radioactive chemical sludges resulting from chemical presipitation treatments, spent ion-exchange resins, all of these originating from nuclear power plants, and sodium nitrate which originate from spent nuclear fuel reprocessing plants. In the case of boiling-water reactors (BWR), the liquid waste originating from the regeneration of ion-exchange resins at a condensate demineralizer, chiefly an aqueous solution of sodium sulfate, is to be solidified in asphalt. In the case of pressurized-water reactors (PWR), the liquid waste containing boric acid from primary coolant is to be solidified in asphalt.
The liquid wastes originating from BWR and PW are usually treated with two evaporators in tandem. The liquid waste from the BWR is concentrated to an aqueous solution of ca. 25% Na.sub.2 SO.sub.4 in the first evaporator whereas the liquid waste from the PWR is concentrated to an aqueous solution of 10-12% sodium borate in the first evaporator. In either case, further concentration is not desirable since it will cause a trouble such as clogged pipes. The concentrated liquid waste is then fed into second evaporator in which it is mixed with asphalt while further evaporation of water is achieved. The mixture of asphalt and the dehydrated waste emerging from the second evaporator is compacted into a predetermined shape and cooled to solidify.
The volume of the BWR liquid waste that has been solidified by the method described above is approximately one quarter of the volume of the feed to the first evaporator, and about one sixth to one eighth in the case of the PWR liquid waste. If the same liquid waste were solidified in concrete, the volume of the final product would be approximately twice the volume of the feed to the first evaporator and this is independent of the type of reactor from which the liquid waste originates. Therefore, if reduction in volume were the only factor to be considered, asphalt would obviously be far better than concrete as a medium for solidifying the liquid waste.
However, as is well know, asphalt which generally has a considerable degree of hardness in cold climates such as in winter will become very soft at elevated temperatures such as in summer. Radioactive wastes in a solidified form desirably have a sufficient hardness to retain their shape even in hot climates such as in midsummer but it is difficult to meet this requirement by simply mixing the liquid waste with asphalt and solidifying the mixture. Because of this drawback, liquid wastes solidified in asphalt have heretofore been considered less stable at elevated temperatures than those solidified in concrete and the method of using asphalt as a solidifying medium has not gained much popularity. As a result of intensive studies conducted in order to solve these problems, the present inventors have successfully developed a method of solidifying radioactive wastes in asphalt that enables continuous mixing with asphalt to be accomplished easily in an evaporator and which provides an asphalt-solidified waste that is stable enough to retain its own shape even in hot climates such as in midsummer.
As already mentioned, mixing radioactive wastes with asphalt and storing the resulting solidified product in repositories is already in commercial practice and the asphalt employed in this method is the straight asphalt specified in JIS K 2207-1980. This straight asphalt has a softening point in the range of 30.degree.-65.degree. C. and an asphalt-solidified radioactive waste containing sodium sulfate or sodium borate in an amount of about 40 wt%, on solids basis, of the mixture has a softening point of about 68.degree. C. as measured by the method specified in JIS K 2207. This asphalt-solified radioactive waste will gradually become fluid and deform if left at ambient temperature. In the current practice, such asphalt-solidified waste is put in a steel container for long-term storage but if holes are made in the container by corrosion or any other reason, the mixture of asphalt and radioactive waste can leak out to contaminate the environment.